Over the past decade, integrated photonics has made important progress in implementing optical and electro-optical devices in silicon for use in various technological applications in fields such as telecommunications, sensing and signal processing. Integrated photonic relies on optical waveguides to implement devices such as optical couplers and switches, wavelength multiplexers and demultiplexers, and polarization splitters and rotators. In particular, integrated photonics based on silicon is a promising candidate for compact integrated circuits due to its compatibility with silicon electronics and standard complementary metal-oxide-semiconductor (CMOS) fabrication methods. The high refractive index contrast between the silicon core and silicon dioxide enables the propagation of highly confined optical modes, which allows scaling integrated photonic waveguides down to submicron level.
One consequence of this high refractive index contrast is that integrated silicon photonic waveguides experience large modal structural birefringence between the two orthogonal transverse electric (TE) and transverse magnetic (TM) fundamental modes of the guided light. Because of this birefringence, integrated photonic waveguides typically exhibit a polarization-dependent behavior. Moreover, since silicon photonic waveguides generally have submicron dimensions and very stringent fabrication tolerance requirements, completely eliminating structural birefringence can prove to be an extremely demanding task.
In order to achieve polarization-independent performance, one may implement a polarization diversity scheme. Generally, polarization diversity is accomplished by using polarization splitters and rotators. In this approach, the two orthogonal TE and TM polarization modes are split in two distinct paths of a polarization diversity circuit. By further rotating the polarization state in one of the paths of the polarization diversity circuit to the orthogonal polarization state, the two paths may be operated in parallel on identical high refractive index contrast waveguide structures. For example, in fundamental-mode silicon waveguides having a certain width and height, it is generally desired to convert the TM polarized signal into a TE polarized signal. Then, as a result of this conversion, only optical functions for the TE modes need to be fabricated and polarization dependence may be eliminated or reduced by using a single polarization (i.e. TE) implementation.
In order for the polarization diversity approach to be practical, on-chip polarization splitters and rotators are desired. However, designing and fabricating integrated waveguide-type polarization rotators can be challenging.
U.S. Pat. No. 7,792,403 to Little et al. (hereinafter LITTLE) discloses a waveguide structure that includes a polarization rotator for rotating the polarization of an electromagnetic signal, preferably by about ninety-degrees. In general, the polarization rotation of the electromagnetic signal by the polarization rotator disclosed in LITTLE is achieved via the geometrical parameters of the polarization rotator. In one embodiment (see, e.g., FIGS. 1 and 2 in LITTLE), the polarization rotator includes an input end, an output end and a midsection extending therebetween and along which polarization rotation is achieved. The midsection has a first and a second level of differing heights and the polarization rotator is referred to as a “bi-level” polarization rotator. The first level of the midsection has a width that decreases along the length of the first level, while the second level has a substantially constant width along the length of the second level.
Waveguide structures such as the one shown in LITTLE can be subject to stringent fabrication tolerances. In particular, it is desirable for the electromagnetic signal to reach the polarization rotation portion in the TM polarization mode in order to be properly rotated. However, vertical taper shapes used to transition between waveguides of different heights can be particularly sensitive to mask alignment during fabrication, and fabrication errors can lead to an undesired pre-rotation of the polarization mode of the guided electromagnetic signal.
There therefore exists a need in the art for an improved polarization rotator assembly for rotating the polarization of light in silicon-based photonic integrated circuits.